The invention relates to a centering device for the manufacture of a center hole in disks, particularly metallized video disks, with, for example, spiral information tracks and an information-free center. A measuring head is employed for the emission of a light beam onto the x-y disk plane and for the reception of the radiation picked up by a detector upon exploitation of the light diffraction at the information tracks, whereby the measuring head and the disk to be centered can be displaced relative to one another in parallel planes.
Optically scanned video disks which rotate with a speed of 1500 or, respectively, 1800 rpm exhibit a center hole into which a shaft for rotation is inserted during the play-back operation. If the center hole is not precisely situated in the center of the disk, then an eccentricity of the informational tracks with respect to the axis of rotation would be effected. Slight eccentricity amounts which are smaller than 50 .mu.m can be compensated by the play-back device. Greater eccentricities can exceed the correction capability of the play-back device and, accordingly, can have a negative influence on the play-back.
Due to the high demands made with respect to a slight eccentricity, pressing the center hole in the same work step as the manufacture and duplication of the disks is not possible. The center hole, therefore, must be produced in an additional work step. To this end, a device for centering the disks with high precision is required.
A method and a device for localizing the center of a circular video disk is known from German AS No. 25 38 383, corresponding to U.S. Pat. No. 3,915,576, incorporated herein by reference. There, a light source for the emission of a light beam onto the surface of the disk to be centered and a light receiver for the acceptance of the light beam diffracted by the informational tracks on the disk are provided. The light receiver is disposed in the optical axis of the light source. The video disk is moved in the disc plane situated perpendicular to said optical axis until the light receiver indicates a diffraction maximum. The localization of the center can be determined from said criterion in such manner that the center of the informational tracks is precisely disposed on the optical axis at a diffraction maximum in the light receiver.
However, this known method fails when the disk to be centered exhibits edge defects, since the measurement of the diffraction maximum is falsified in this case due to the light beam striking the edge area of the disk.